In a gas chromatograph, before the introduction of a sample into a separation column which performs the separation of components, it is general practice to concentrate the sample by use of a capillary column or a packed column and to increase the analysis sensitivity of a component to be analyzed. In the introduction of a sample into a gas chromatograph, the cold on-column injection method and the programmed temperature vaporization method (the PTV method) are used. In a case where a gas chromatograph-mass spectrometer (GC/MS) is used or in a case where a detector, such as a hydrogen flame ionization detector (FID), is used as the detector of a gas chromatograph, in a case where in introducing components which have eluted from an analysis column into the ionization chamber of a mass spectrograph or the hydrogen flame portion of a hydrogen flame ionization detector, and in a case where a gaseous sample and the like are transferred to keep the column warm, it is general practice to use a tube which is heated so that the condensation of the gas does not occur, i.e., a heat tube.
As methods of concentrating and collecting samples in a gas chromatograph, there are available a method which involves feeding a sample into a packed column which is packed with a filler which selectively adsorbs and collects a component to be analyzed in a sample, causing the filler to adsorb and collect the component to be analyzed, and heating thereafter the column, thereby causing the component to be analyzed to be desorbed from the filler, a method which involves feeding a sample into a cooled column, aggregating the component to be analyzed in the sample by causing the component to be adsorbed and condensed on an inner wall of the column, and heating the column thereafter, whereby the component to be analyzed is vaporized and desorbed at a high speed, and the like.
And as methods of heating this column, there are available, for example, first as shown in FIG. 11, a method which involves winding an insulated heater tube 90 like a sheathed heater directly on a tube 91 to be heated (hereinafter briefly called a tube 91), such as a column, thereby to heat the tube, second as shown in FIG. 12, a method which involves using a double construction tube consisting of a tube 91 and an outer tube 92 formed around this tube and heating the tube by introducing a high-temperature gas, such as the heated air, into the space formed between the outer tube 92 and the tube 91, third as shown in FIG. 13, which involves using a direct heating tube, by which electrodes 93, 93 are provided at both ends of a tube 91 and the tube 91 is heated by causing a DC current or an AC current to flow directly through the tube 91, and fourth as shown in FIG. 14, a method which involves inserting a heater 95 and a sensor 96 along with a tube 91 into a heating block 94 made of aluminum, brass and the like and performing heating, whereby the inserted tube 91 is heated and the temperature of the tube is kept, these methods being disclosed for example in Japanese Patent Laid-open No. Heisei 5-502734 and Japanese Patent Laid-Open No. 6-222048.
Heating methods of tube similar to those given above are used also in a case where a gas chromatograph-mass spectrometer (GC/MS) is used or in a case where a detector, such as a hydrogen flame ionization detector (FID), is used, in a heat tube which is used in the transfer of a sample from the column of the gas chromatogram to the mass spectrometer and to the detector, such as a hydrogen flame ionization detector (FID), or in a column and a vaporization chamber in various methods of introducing samples of a gas chromatograph.
However, these conventional methods of heating a column, a heat tube and the like have had the following problems. Although the first method can be very easily carried out, for example, when as in the case of a cryotrap used in a gas chromatograph, cooling and heating are alternately performed and the temperature change of the cryotrap is severe, the electrical insulation of a heater may sometimes be broken, thus involving risk. Therefore, it is necessary to select and use a heater having a sufficient insulation distance and safe watt density in terms of design, with the result that the rate at which the tube is heated may not be sufficient. As shown in FIG. 15, this heating rate has a great effect on the shape of a chromatogram peak. That is, the higher the temperature rise rate, the narrower the sample band, thereby making it possible to detect the sample with high sensitivity, and the lower the temperature rise rate, the wider the sample band, thereby making it impossible to detect the sample with high sensitivity.
Also the second method has the greatest weak point that the heating rate is low in the same manner as the first method. The reason is as follows. That is, because the specific heat capacity of gases is very small, it is necessary to cause a large volume of a high-temperature gas to flow at a time if rapid heating is required. However, in order to realize this, large-scale equipment becomes necessary and the manufacturing cost also rises.
In the third method, very high heating rates can be obtained by causing a current to flow directly through a tube 91 without the use of a heater. However, in the conventional direct heating method, heat mass is present in electrode portions at both ends and, therefore, this poses the problem that there are low-temperature areas, which are what is called cold spots, in both end portions. In order to avoid cold spots, there have thitherto been adopted measures such as adding heating portions in both ends to keep warm the temperature of the two ends. In connecting the electrodes 93 to a power supply section, materials having small electric resistance, such as nickel wires and copper wires, are used. In order to minimize the heat mass of the electrodes 93, very complicated assembling has been performed; for example, electric wires are welded or brazed directly to the tube.
The fourth method can be performed very easily and is often used in the sample introduction portion of a gas chromatograph. However, much time is required before the sample introduction portion is heated because of a large thermal capacity and inversely when cooling is performed, much time is required. Therefore, this fourth method is in adaptable to the cold injection method, which has recently begun to be frequently used. When used in the introduction portion to a detector, such as a hydrogen flame ionization detector, it is desirable that a collector portion be in a cooled condition. However, when the fourth method is used, even the collector portion is heated, and the oven of a gas chromatogram is also heated. Thus, the fourth method has exerted an undesirable influence on a detector, an oven and the like.
Therefore, in order to solve the above-described conventional problems, it is an aspect of the present invention to provide a direct heating tube which has a sufficient heating rate and a sufficient cooling rate, and has no cold spots therein, making it possible to ensure a uniform temperature distribution in the whole part thereof or a temperature distribution having a desired temperature gradient, and making it possible to keep constant the temperature of a fluid which is caused to flow through the tube or to give a desired change to the temperature of the fluid. Also, the present invention has as an aspect the provision of a direct heating tube which does not exert an adverse influence on devices near the tube, such as a detector and an oven, even by heating the tube, and a direct heating tube of simple construction which is capable of being manufactured at low cost. Also, the present invention has as its object the provision of a direct heating tube which permits designs in which the ease of assembling is considered for an electrode portion. Furthermore, the present invention has an object to provide a heating method which keeps constant the temperature of a fluid which is caused to flow through a tube or gives a desired change to the temperature of the fluid.